Its mainly because they're 'cowards' Both races would jump at the chance to manipulate people/things into helping them reach their final objective, and there's no doubt the orks/necrons would kill the ig cowards AFTER they're done.

the game n the idea is fantastic! but the story line was rushed! i liked the storylines an cutscense from the previous 2 games. the fact that it didnt really tell a " STORY" just got a bit anoyyin reli but oh well